The present invention generally relates to a continuously variable transmission, and more specifically to a method and apparatus for adjusting transmission ratio commands for a continuously variable transmission.
Many work machines, particularly earth working machines, use a continuously variable transmission to drive traction wheels or tracks which propel the work machine. Typically, a hydrostatic transmission, consisting of a variable speed hydraulic pump and a hydraulic motor, provides a continuously variable speed output to the wheels or tracks of the work machine. In particular, the speed output can be continuously varied by controlling the displacement of the hydraulic pump.
Conventional transmissions operate over a distinct number of gear ratios. Shifts between adjacent gear ratios generally keeps an engine, which drives the transmission, within its desired operating range. A drawback to a continuously variable transmission is that transmission ratios, or the ratio of transmission output speed to engine speed, can be commanded which are outside of the desired operating range of the engine which drives the transmission. If a transmission ratio is too large, then the engine may run below its desired operating point or lug limit. Therefore, commanding too high of a transmission ratio may cause the engine to stall. On the other hand, if the transmission ratio is too low, the engine may run at a speed well above its overspeed limit. Operating the engine above its overspeed limit can potentially damage the engine or drivetrain.
What is needed therefore is a method and apparatus for adjusting the transmission ratio commands for a continuously variable transmission which overcomes the above-mentioned drawbacks.
In accordance with a first aspect of the present invention, there is provided an apparatus for controlling a continuously variable transmission. The apparatus includes (i)an input shaft driven by an engine, (ii)an output shaft, (iii) an operator input for generating speed commands, and (iv) a controller operable to receive said speed commands and generate transmission ratio commands which control a ratio of a speed of the output shaft to a speed of the input shaft. An upper transmission ratio limit is defined by first engine speed and a lower transmission ratio limit is defined by a second engine speed. When the speed command requires a transmission ratio command greater than the upper transmission ratio limit, the controller adjusts the transmission ratio command to the upper transmission ratio limit, whereas when the speed command requires a transmission ratio command less than the lower transmission ratio limit the controller adjusts the transmission ratio command to the lower transmission ratio limit.
In accordance with a second aspect of the present invention, there is provided a method for controlling a continuously variable transmission. The transmission includes (i) an input shaft driven by an engine, (ii)an output shaft, (iii)an operator input for generating speed commands, and (iv) a controller operable to receive said speed commands and generate transmission ratio commands which control a ratio of a speed of the output shaft to a speed of the input shaft. The method includes the steps of defining an upper transmission ratio limit with a first engine speed and defining a lower transmission ratio limit with a second engine speed. The method further includes the steps of adjusting the transmission ratio command to the upper transmission ratio limit when the speed command requires a transmission ratio command greater than the upper transmission ratio limit and adjusting the transmission ratio command to the upper transmission ratio limit when the speed command requires a transmission ratio command less than the lower transmission ratio.